So many people think the brain is just filled with neurons, but that is so untrue. There are at least six major classes of cells that reside in our brains and throughout our nervous system.
Here is a list of them
I bring this list up because there could be potential cancer in any one of them, but the worst and by far the deadliest is what happens to astrocytes. Glioblastoma is the disease that affects this type of cell and has as much of a 7% survival rate with many dying from it. Now I don't know which cancer he had, but he had to have a rare one to get it at such a young age (except if he lived half of his life on the Mississippi River in Russia) and one that was deadly for his body succumb to it at an age when his immune system was as good as ever.